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[Bug dynamic-link/21258] New: Branch predication in _dl_runtime_resolve_avx512_opt leads to lower CPU frequency
- From: "hjl.tools at gmail dot com" <sourceware-bugzilla at sourceware dot org>
- To: glibc-bugs at sourceware dot org
- Date: Thu, 16 Mar 2017 19:19:29 +0000
- Subject: [Bug dynamic-link/21258] New: Branch predication in _dl_runtime_resolve_avx512_opt leads to lower CPU frequency
- Auto-submitted: auto-generated
https://sourceware.org/bugzilla/show_bug.cgi?id=21258
Bug ID: 21258
Summary: Branch predication in _dl_runtime_resolve_avx512_opt
leads to lower CPU frequency
Product: glibc
Version: 2.26
Status: NEW
Severity: normal
Priority: P2
Component: dynamic-link
Assignee: unassigned at sourceware dot org
Reporter: hjl.tools at gmail dot com
Target Milestone: ---
Target: x86-64
On Skylake server, _dl_runtime_resolve_avx512_opt is used to preserve
the first 8 vector registers. The code layout is
if only %xmm0 - %xmm7 registers are used
preserve %xmm0 - %xmm7 registers
if only %ymm0 - %ymm7 registers are used
preserve %ymm0 - %ymm7 registers
preserve %zmm0 - %zmm7 registers
Branch predication always executes the fallthrough code path to preserve
%zmm0 - %zmm7 registers speculatively, even though only %xmm0 - %xmm7
registers are used. This leads to lower CPU frequency.
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